Struct up the expressions collector

Summary:
In review, we noted that some of the functions were starting to have
too many arguments for readability. Making the AST traversal state a struct
cuts down on boilerplate a bit.

Reviewed By: yangdanny97

Differential Revision: D96940766

fbshipit-source-id: c0b2aad57fb317f901053dbff0aa33bfd923c4b6

Author: stroxler